Tailoring Alkalized and Oxidized V2CTx as Anode Materials for High-Performance Lithium Ion Batteries

V2CTx MXenes have gained considerable attention in lithium ion batteries (LIBs) owing to their special two-dimensional (2D) construction with large lithium storage capability. However, engineering high-capacity V2CTx MXenes is still a great challenge due to the limited interlayer space and poor surface terminations. In view of this, alkalized and oxidized V2CTx MXenes (OA-V2C) are envisaged. SEM characterization confirms the accordion-like layered morphology of OA-V2C. The XPS technique illustrates that undergoing alkalized and oxidized treatment, V2CTX MXene replaces -F and -OH with -O groups, which are more conducive to pseudocapacitive properties as well as Na ion diffusion, providing more active sites for ion storage in OA-V2C. Accordingly, the electrochemical performance of OA-V2C as anode materials for LIBs is evaluated in this work, showing excellent performance with high reversible capacity (601 mAh g−1 at 0.2 A g−1 over 500 cycles), competitive rate performance (222.2 mAh g−1 and 152.8 mAh g−1 at 2 A g−1 and 5 A g−1), as well as durable long-term cycling property (252 mAh g−1 at 5 A g−1 undergoing 5000 cycles). It is noted that the intercalation of Na+ ions and oxidation co-modification greatly reduces F surface termination and concurrently increases interlayer spacing in OA-V2C, significantly expediting ion/electron transportation and providing an efficient way to maximize the performance of MXenes in LIBs. This innovative refinement methodology paves the way for building high-performance V2CTx MXenes anode materials in LIBs.


Introduction
Currently, lithium ion batteries (LIBs) are undoubtedly considered the main energy storage option for portable electronics, electromotives and stationary smart grids.As we all know, graphite is regarded as the common anode material in LIBs because of its low cost and satisfactory voltage plateau.However, the limited capacity severely impedes the energy density of the whole battery.Therefore, exploiting high-capacity anode materials with steady electrochemical behavior is beneficial to realizing high-energy-density LIBs.Transition metal carbides and nitrides (MXenes), as a family of 2D layered structures, have been fully investigated since 2011 [1,2].MAX and M n+1 AX n phases indicate hexagonal layered transition metal carbides and nitrides.Commonly, MXenes can be obtained via etching the A-element from the layered MAX phase [3][4][5].The obtained layered MXenes have a general chemical formula, which can be denoted as M n+1 X n T x , where M is a transition metal element (Ti, V, etc.), X is a carbon or nitrogen element, n = 1, 2 or 3, and T x stands for adsorbed functional groups at the surface such as -F, -O and -OH [6][7][8].MXenes generally possess high electronic conductivity, excellent mechanical behavior, high surface area and large interlayer spacing, as well as rich surface functional groups [9][10][11][12][13][14].In this regard, MXenes show great application prospects in rechargeable batteries [15], supercapacitors [16], photocatalysts [17], catalysts [18] Materials 2024, 17, 3516 2 of 12 and other fields.Among the MXenes family, most of the exploitations focus on Ti 3 C 2 T x , mainly because of its mature preparation and stripping method [19][20][21][22].Compared with Ti 3 C 2 T x , V 2 CT x not only occupies better conductivity but also shows higher capacity and lower ion transport barrier [23].As the anode material in LIBs, the theoretical capacity of V 2 CT x can reach as high as 940 mAh g −1 , which is much higher than that of other MXenes [24].However, it is still a huge challenge to completely remove the functional groups such as -F and -OH of the V 2 CT x , which largely hinders the transmission of Li + ions and reduces its capacity [25][26][27][28][29][30].
In order to ameliorate the electrochemical performance of V 2 CT x , it is necessary to reasonably enhance its reaction kinetics and ion transport efficiency.For example, Wang et al. reported the intercalation of Co 2+ into hydrated bilayers of V 2 C MXene to expand its interlayer spacing and form a stable V-O-Co bond between the layers, which considerably improves the performance of V 2 C MXene with a high specific capacity of 1117.3 mAh g −1 [31].Similarly, Sn 4+ -decorated V 2 C MXene demonstrated improved performance, as well as exceptional rate and cyclic stability in LIBs due to the expanded interlayer spacing and the creation of V-O-Sn bonds.Recently, a co-modification of V 2 CT X MXene with K + ions and -O functional groups was proposed.Notably, K + ions were introduced to stabilize the interlayer structure and prevent nanosheet aggregation, while the -O terminal group was selectively added to the MXene surface to enhance the reversible capacity for Li + storage.The co-modified V 2 CT X MXene exhibits superior reversible specific capacities of 671.8 mA h g −1 at 0.1 A g −1 and 318 mA h g −1 at 1.0 A g −1 [32].Additionally, Ca 2+ ions were introduced into the interlayer of V 2 CT X MXenes through alkalization and ion-exchange processes.Structural and chemical analyses revealed modifications in surface terminations and interlayer spacing.Calcium ion intercalation resulted in a larger interlayer distance, facilitating efficient Li + transport channels and providing more space for ion storage.Furthermore, the ion-exchange process reduced F surface terminations, enhancing electron conductivity and activating the surface terminations of V 2 CT X MXenes for participation in redox reactions.Thus, adjusting the layer structure of V 2 CT X via cation intercalation and control techniques significantly promotes ion diffusion kinetics and boosts surface electrochemical reaction activity, leading to superior electrochemical performance [33][34][35][36][37].
Inspired by the previous works, here, the alkalized and oxidized V 2 CT x (OA-V 2 C) was prepared based on a hydrothermal reaction.Abundant O-terminal groups on the surface after oxidation effectively improve the diffusion kinetics and structural stability of Li + .The surface terminations, such as hydroxyl (-OH), oxygen (-O), or fluorine (-F), can significantly increase the electrochemical activity.These functional groups could supply additional sites for Li ion storage, leading to higher capacity.Concurrently, the Na + intercalation effectively enlarges the interlayer of V 2 CT x .Therefore, the OA-V 2 C electrode delivers enhanced capacity and excellent electrochemical performance, showing excellent performance with high reversible capacity (601.0 mAh g −1 at 0.2 A g −1 over 500 cycles) and competitive rate performance (222.2 mAh g −1 and 152.8 mAh g −1 at 2 A g −1 and 5 A g −1 ) as well as durable long-term cycling property (252 mAh g −1 at 5 A g −1 undergoing 5000 cycles).

Material Preparation
Multilayered V 2 CT x (M-V 2 C) was obtained based on a modified etching method [1,2].In order to obtain M-V 2 C, 1 g NaF powders purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China) were added into a 100 mL glass beaker, followed by adding 10 mL HCl (30%) purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China) and deionized water with a volume ratio of 1:1.Then, 1 g V 2 AlC (Nanjing XFNANO Materials Tech Co., Ltd, Nanjing, China) powders were slowly added to the above mixed solution and stirred for 30 min.Subsequently, the mixture was transferred into a 100 mL Teflon-lined stainless-steel autoclave and heated at 90 • C for 7 days.The resulting solution was centrifuged and washed repeatedly until the pH ≈ 7 and then freeze-dried to acquire M-V 2 C. The oxidized V 2 C MXene (O-V 2 C) was fabricated by the hydrothermal method.Specifically, 0.2 mL H 2 O 2 (30%) was mixed with 30 mL deionized water under stirring for 5 min at room temperature.Then, the prepared M-V 2 C (0.2 g) was added to the mixture and stirred for 30 min.The obtained solution was then transferred into a Teflon-lined stainless-steel autoclave and heated at 140 • C for 24 h.Subsequently, the resultant product was washed with deionized water and alcohol several times to acquire O-V 2 C.An amount of 0.2 g M-V 2 C was added into 1 M NaOH solution (30 mL) and stirred for 30 min at room temperature.Subsequently, the acquired solution was transferred into a Teflon-lined stainless-steel autoclave and underwent a hydrothermal process at 140 • C for 24 h.Then, the obtained black sample was washed with deionized water and alcohol several times to acquire alkalized V 2 C MXene (A-V 2 C).The prepared M-V 2 C (0.2 g) was added into 1 M NaOH solution (30 mL) with 0.2 mL H 2 O 2 and stirred for 30 min at room temperature.Then, it was transferred into a Teflon-lined stainless-steel autoclave and heated at 140 • C for 24 h.The product was washed with deionized water and alcohol several times, then dried in the oven at 60 • C for 12 h to obtain the oxidized and alkalized V 2 C MXene (OA-V 2 C).

Material Characterizations
To determine the external morphologies of the samples, scanning electron microscopy (SEM, JEOL, Tokyo, Japan, JSM-7500F) was performed.X-ray diffraction spectra were acquired via the JEOL-Smart Lab facility.Chemical states were identified by X-ray photoelectron spectroscopy (XPS, Shimadu, AXIS Supra, Kyoto, Japan).An aluminum Kα X-ray source delivering a photon energy of 1486.6 eV served as the excitation mechanism.XPS PEAK4.1 software was used to fit the XPS date employing the C 1s peak at 284.8 eV for adventitious carbon as a reference.The Shirley background subtraction method was employed to fit the XPS data.

Electrochemical Characterizations
The electrochemical properties were evaluated using CR2032 coin half cells.Working electrodes were prepared from a mixture consisting of 70 wt% active materials, 20 wt% acetylene black and 10 wt% polyvinylidene fluoride (PVDF) dissolved in N-methyl-2pyrrolidone (NMP).This slurry was then coated onto copper foil and dried in a vacuum at 60 • C for 16 h.The electrode mass loading was 2 mg cm −2 .Lithium foil served as the counter electrode, and glass fiber acted as the separator.The electrolyte used was 1 M LiPF 6 in a solution of ethylene carbonate (EC) and dimethyl carbonate (DEC) in a 1:1 volume ratio, with 5% fluoroethylene carbonate (FEC) added.The battery assembly took place in an argon-filled glove box, maintaining H 2 O and O 2 concentrations below 0.01 ppm.A battery testing system (CT2001, LANHE, Guangzhou, China) assessed the cycling and rate performance within a voltage range of 0.01 to 3.0 V. Cyclic voltammetry (CV) was performed using a CHI 760E electrochemical workstation between 0.01 and 3.0 V. Electrochemical impedance spectroscopy (EIS) was also carried out using the same workstation, spanning from 100 kHz to 0.01 Hz.

Results and Discussion
The synthesis process and structure of OA-V 2 C are schematically shown in Figure 1a.It needs to be mentioned that the V 2 AlC MAX displayed a typical dense microstructure (Figure S1a), while the accordion shape of V 2 C MXenes was obtained by selectively etching the Al layers in the V 2 AlC MAX precursor via the mixed solution of HCl and NaF (Figure S1b,c).Afterwards, the obtained multi-layered V 2 C MXenes (M-V 2 C) were added into the mixed solution of NaOH and H 2 O 2 for a hydrothermal reaction to acquire Na + intercalated and oxidized V 2 C MXenes (OA-V 2 C) [38].Based on the SEM images in Figure 1b-d and Figure S1b,c, the M-V 2 C, A-V 2 C, O-V 2 C and OA-V 2 C show a similar accordion-like layered morphology, which is beneficial to the rapid Li + diffusion.TEM-EDS mapping (Figure 1e) shows that V, O, C and Na elements are evenly distributed in OA-V 2 C, demonstrating the successful pre-intercalation of Na + in OA- 2a.Compared with the diffraction peaks of V 2 AlC, the (002) peaks of M-V 2 C, A-V 2 C, O-V 2 C and OA-V 2 C moved left after the modification, and the characteristic reflection of V 2 AlC at 13.3 • and 41.2 • almost disappeared, revealing that the Al element was selectively etched from V 2 AlC and the ultimate formation of V 2 C MXenes.In addition, there were still some impurities such as vanadium and aluminum oxides presented after the etching process for the derived four specimens, which is similar to previous reports [39,40].
In order to investigate the chemical compositions and bond states of M-V 2 C, A-V 2 C, O-V 2 C and OA-V 2 C, XPS spectra are presented in Figure 2b-e and Figure S2.It can be found that the V, O, C and Na elements in OA-V 2 C based on the whole XPS survey (Figure 2b) are consistent with the above EDS elemental mappings.There are three pair deconvoluted peaks at 523.5/516.5, 522.6/514.9,521.4/513.8eV which can be ascribed to V 4+ , V 3+ and V 2+ in OA-V 2 C (Figure 2c) [39,40].The appearance of V 4+ peaks can be attributed to vanadium oxide, indicating that V 2 CT X MXene is inevitably slightly oxidized during etching, as well as alkalized and oxidized treatment.The peaks at 531.4, 529.8 and 533.8 eV in the high-resolution O 1s XPS spectrum (Figure 2d) can be assigned to V-OH, V-O and H-O-H bonds, indicating the substitution of oxygen-containing groups.Through alkalized and oxidized treatment, V 2 CTX MXene substitutes -F and -OH groups with -O groups.These -O substitutions enhance pseudocapacitive properties and facilitate Na ion diffusion, resulting in more active sites available for ion storage in OA-V 2 C. Figure 2e displays the high-resolution C 1s spectrum, which can be deconvoluted into four peaks at 288.5, 286.3, 284.8 and 282.5 eV, corresponding to the C=O, C-O, C-C and C-V bonds [41].The atomic percentages of F and O in M-V 2 C, A-V 2 C, O-V 2 C and OA-V 2 C based on the XPS results prove the reduced F surface termination and increased O-terminate groups in O-V 2 C, A-V 2 C and OA-V 2 C (Table S1).Most importantly, the OA-V 2 C occupies the highest content of oxygen, illustrating that the alkalization and oxidation co-modification could greatly reduce F surface termination and concurrently increase O-terminate groups in OA-V 2 C. The composition of the samples with a few Al certifies that highly pure V 2 C MXene was successfully obtained.
The electrochemical performances of OA-V 2 C, A-V 2 C, O-V 2 C and M-V 2 C anodes were tested in LIBs (Figure 3). Figure 3a-d represent the galvanostatic charge-discharge curves of the initial three cycles for the M-V 2 C, OA-V 2 C, A-V 2 C and O-V 2 C anodes, respectively.The M-V 2 C shows the lowest charge-discharge capacity in the initial cycle with an initial Coulombic efficiency (ICE) of 27.6% at 0.2 A g −1 , which might be attributed to the limited interlayer spacing and unsatisfied electrical conductivity of M-V 2 C. Conversely, the OA-V 2 C delivered charge-discharge capacity of 798.9/1414 mAh g −1 with the highest ICE of 56.5%.The A-V 2 C anode exhibited a charge-discharge capacity of 721.5/382.3mAh g −1 with an ICE of 53.0%, and the O-V 2 C anode displayed a charge-discharge capacity of 820.1/443.2mAh g −1 with an ICE of 54.0%The irreversible capacity loss in the first cycle for the four electrodes could be attributed to the formation of a solid electrolyte interface (SEI) layer [42][43][44].The charge-discharge curves of the OA-V 2 C anode at various current densities in Figure S3 with low potential polarization validate the excellent electrochemical kinetics.Figure 3e-h display the CV curves of the M-V 2 C, OA-V 2 C, A-V 2 C and O-V 2 C anodes at 0.1 mV s −1 in the initial three cycles.In the first cathodic scan, the obvious reduction peaks weakened or even vanished for the four samples.The increase in Oterminated groups on the modified V 2 CT x surface may explain why there is an enhanced ability to uptake Li + .In the following cycles, the CV curves nearly overlap, indicating the excellent reversibility of the modified V 2 CT x electrodes [45].Figure 3i displays the cycling performance of the four electrodes at 0.2 A g −1 .It can be observed that the OA-V 2 C possesses a reversible capacity of 601 mAh g −1 underneath 500 cycles at 0.2 A g −1 corresponding to a Coulombic efficiency of 100%.In contrast, the A-V 2 C, O-V 2 C and M-V 2 C electrodes just delivered capacities of 389, 502 and 71 mAh g −1 undergoing 500 cycles.It is noted that the lithium ion storage capability of OA-V 2 C is significantly improved after alkalization and oxidation co-modification.The increased -O groups and the interlayer intercalation of Na + ions enlarges the interlamellar space and increases the active sites of the material, which is convenient for more Li + ion insertion in the MXene layer.The -O functional group modification could improve the Li + ion storage performance undergoing the adjustment.OA-V 2 C typically have various surface terminations, such as hydroxyl (-OH), oxygen (-O) or fluorine (-F), which can significantly increase their electrochemical activity.These functional groups could supply additional sites for ion storage, leading to higher capacities.Furthermore, OA-V 2 C demonstrate pseudocapacitive behavior, where charge storage occurs not only through conventional faradaic reactions but also via surface or near-surface redox reactions.This can contribute to higher charge storage capacities by leveraging fast redox reactions at or near the electrode surface.Defects and vacancies within the MXene structure can act as additional active sites for ion accommodation or adsorption, further increasing the charge storage capacity.
Coulombic efficiency of 100%.In contrast, the A-V2C, O-V2C and M-V2C electrodes just delivered capacities of 389, 502 and 71 mAh g −1 undergoing 500 cycles.It is noted that the lithium ion storage capability of OA-V2C is significantly improved after alkalization and oxidation co-modification.The increased -O groups and the interlayer intercalation of Na + ions enlarges the interlamellar space and increases the active sites of the material, which is convenient for more Li + ion insertion in the MXene layer.The -O functional group modification could improve the Li + ion storage performance undergoing the adjustment.OA-V2C typically have various surface terminations, such as hydroxyl (-OH), oxygen (-O) or fluorine (-F), which can significantly increase their electrochemical activity.These functional groups could supply additional sites for ion storage, leading to higher capacities.Furthermore, OA-V2C demonstrate pseudocapacitive behavior, where charge storage occurs not only through conventional faradaic reactions but also via surface or near-surface redox reactions.This can contribute to higher charge storage capacities by leveraging fast redox reactions at or near the electrode surface.Defects and vacancies within the MXene structure can act as additional active sites for ion accommodation or adsorption, further increasing the charge storage capacity.   Figure 4a shows the rate performance of the four samples.The OA-V 2 C anode possesses the most competitive rate capability with discharge capacities of 567.7, 474.8, 370.2, 291.4,222.2 and 152.8 mAh g −1 at current densities of 0.1, 0.2, 0.5, 1, 2 and 5 A g −1 , respectively.As the current density returns to 0.2 A g −1 , the reversible capacity recovers to 474.3 mAh g −1 .In contrast, the M-V 2 C, A-V 2 C and O-V 2 C electrodes just deliver capacities of 98.3, 27.3 and 47.2 mAh g −1 at 5 A g −1 .To further investigate the electrochemical behavior of OA-V 2 C anode, the electrochemical impedance spectra (EIS) of M-V 2 C, OA-V 2 C, A-V 2 C and O-V 2 C after 500 cycles are displayed in Figure 4b with the equivalent circuit model in the inset.It can be seen that the charge-transfer resistance (R ct ) of OA-V 2 C (94.6 Ω) is the smallest among the four samples.The R ct values are estimated to be 269.2,257.4 and 463.2 Ω for the A-V 2 C, O-V 2 C and M-V 2 C samples, respectively.It is noted that the Na pre-intercalation and generated surface oxygen groups could effectively improve the electronic conductivity after the alkalization and oxidation treatment.Long-term cycling performance of the four specimens at 5 A g −1 for 5000 cycles is offered in Figure 4c.The OA-V 2 C electrode shows the most excellent stability with a high capacity of 252 mAh g −1 remained after 5000 prolonged cycles.Even after 5000 cycles, the capacity keeps increasing, while the capacity of A-V 2 C and O-V 2 C rapidly declines after 2500 cycles.The M-V 2 C anode delivered an ultralow capacity at 5 A g −1 which might be due to the poor electrical conduction and moderate interlayer spacing.A comparison of electrochemical performances between OA-V 2 C and various MXene-based anode materials in LIBs is displayed in Table S2, disclosing the competitive performance of the OA-V 2 C [40,[45][46][47][48][49][50].
To further investigate the rapid Li + storage kinetics of the OA-V 2 C electrode, CV curves at various scan rates ranging from 0.1 to 2 mV s −1 are recorded, as shown in Figure 4d.Typically, the relationship between the scan rate (v) and the peak current (i) can be described as follows: Materials 2024, 17, x FOR PEER REVIEW 6 of 12 Ω for the A-V2C, O-V2C and M-V2C samples, respectively.It is noted that the Na pre-intercalation and generated surface oxygen groups could effectively improve the electronic conductivity after the alkalization and oxidation treatment.Long-term cycling performance of the four specimens at 5 A g −1 for 5000 cycles is offered in Figure 4c.The OA-V2C electrode shows the most excellent stability with a high capacity of 252 mAh g −1 remained after 5000 prolonged cycles.Even after 5000 cycles, the capacity keeps increasing, while the capacity of A-V2C and O-V2C rapidly declines after 2500 cycles.The M-V2C anode delivered an ultralow capacity at 5 A g −1 which might be due to the poor electrical conduction and moderate interlayer spacing.A comparison of electrochemical performances between OA-V2C and various MXene-based anode materials in LIBs is displayed in Table S2, disclosing the competitive performance of the OA-V2C [40,[45][46][47][48][49][50].To further investigate the rapid Li + storage kinetics of the OA-V2C electrode, CV curves at various scan rates ranging from 0.1 to 2 mV s −1 are recorded, as shown in Figure 4d.Typically, the relationship between the scan rate (v) and the peak current (i) can be described as follows:   A b value of 0.5 suggests that the capacity is governed by the ion diffusion process, whereas a b value of 1.0 indicates a capacitive-controlled process [43,44].Subsequently, according to Equation (2), the slopes of four redox peaks are calculated to be 0.92, 0.91, 0.79 and 0.83, respectively (Figure 4e).This result shows that the Li + insertion/extraction in OA-V2C anode can be attributed to both capacitive and diffusion behaviors.In addition, the capacitive (k1v) and diffusion (k2v 1/2 ) contributions in CV curves are determined by subsequent Equation (3) [51][52][53].
The capacitive contribution is assessed to be 54.5% at 0.5 mV s −1 for the OA-V2C anode (Figure 4f).The capacitive contribution ratios at different scan rates are displayed in Fig- ure 4g, in which the capacitive contribution ratios rise from 54.5% to 74.8% with the increase in the sweep rates.All of these results show that the capacitive charge storage plays an important role in the capacity of OA-V2C, which is the reason for the excellent chargetransfer kinetics [54][55][56][57].The ex situ XRD patterns during the charge-discharge process in Figure 5a confirms the (de)intercalation mechanism of Li + in the OA-V2C electrode.Upon discharge, the (002) peaks of OA-V2C shift left, caused by the increased distance of the interlayer.Conversely, the (002) peaks of OA-V2C shift right during the charging process, illustrating the highly reversible property of OA-V2C.Most importantly, the layered architecture can still be maintained even after 5000 cycles (Figure 5b), illustrating the excellent structural stability of OA-V2C.This innovative refinement methodology paves the way for building high-performance V2CTx MXenes anode materials in LIBs.A b value of 0.5 suggests that the capacity is governed by the ion diffusion process, whereas a b value of 1.0 indicates a capacitive-controlled process [43,44].Subsequently, according to Equation (2), the slopes of four redox peaks are calculated to be 0.92, 0.91, 0.79 and 0.83, respectively (Figure 4e).This result shows that the Li + insertion/extraction in OA-V 2 C anode can be attributed to both capacitive and diffusion behaviors.In addition, the capacitive (k 1 v) and diffusion (k 2 v 1/2 ) contributions in CV curves are determined by subsequent Equation (3) [51][52][53].
The capacitive contribution is assessed to be 54.5% at 0.5 mV s −1 for the OA-V 2 C anode (Figure 4f).The capacitive contribution ratios at different scan rates are displayed in Figure 4g, in which the capacitive contribution ratios rise from 54.5% to 74.8% with the increase in the sweep rates.All of these results show that the capacitive charge storage plays an important role in the capacity of OA-V 2 C, which is the reason for the excellent chargetransfer kinetics [54][55][56][57].The ex situ XRD patterns during the charge-discharge process in Figure 5a confirms the (de)intercalation mechanism of Li + in the OA-V 2 C electrode.Upon discharge, the (002) peaks of OA-V 2 C shift left, caused by the increased distance of the interlayer.Conversely, the (002) peaks of OA-V 2 C shift right during the charging process, illustrating the highly reversible property of OA-V 2 C. Most importantly, the layered architecture can still be maintained even after 5000 cycles (Figure 5b), illustrating the excellent structural stability of OA-V 2 C.This innovative refinement methodology paves the way for building high-performance V 2 CT x MXenes anode materials in LIBs.

Conclusions
In summary, OA-V 2 C, O-V 2 C, A-V 2 C and M-V 2 C samples were prepared via a hydrothermal reaction and evaluated as anode materials in LIBs in this work.Among the four samples, the OA-V 2 C demonstrates the most excellent performance among the four specimens with a high reversible capacity (601 mAh g −1 at 0.2 A g −1 after 500 cycles), excellent rate performance (222.2 mAh g −1 and 152.8 mAh g −1 at 2 A g −1 and 5 A g −1 , respectively) and stable long-term cycling performance (252 mAh g −1 at 5 g −1 after 5000 cycles).It is noted that the oxidation modification greatly reduces the -F group and improves the reversible capacity.On the other hand, the introduction of Na + into V 2 CT x MXenes significantly stabilizes interlayer construction and hinders the aggregation of nanosheets, expediting charge transfer and Li + diffusion efficiency.It is noted that the surface terminations, such as hydroxyl (-OH), oxygen (-O) or fluorine (-F), can significantly increase the electrochemical activity.These functional groups could supply additional sites for Li ion storage, leading to higher capacity.Furthermore, OA-V 2 C demonstrate pseudocapacitive behavior, where charge storage occurs not only through conventional faradaic reactions but also via surface or near-surface redox reactions.This can contribute to higher charge storage capacities by leveraging fast redox reactions at or near the electrode surface.Defects and vacancies within the MXene structure can act as additional active sites for ion accommodation or adsorption, further increasing the charge storage capacity.This special modification for V 2 CT x MXenes in this work provides an efficient pathway for building high-performance anode materials in LIBs.

Figure
Figure 4a shows the rate performance of the four samples.The OA-V2C anode possesses the most competitive rate capability with discharge capacities of 567.7, 474.8, 370.2, 291.4,222.2 and 152.8 mAh g −1 at current densities of 0.1, 0.2, 0.5, 1, 2 and 5 A g −1 , respectively.As the current density returns to 0.2 A g −1 , the reversible capacity recovers to 474.3 mAh g −1 .In contrast, the M-V2C, A-V2C and O-V2C electrodes just deliver capacities of 98.3, 27.3 and 47.2 mAh g −1 at 5 A g −1 .To further investigate the electrochemical behavior of OA-V2C anode, the electrochemical impedance spectra (EIS) of M-V2C, OA-V2C, A-V2C and O-V2C after 500 cycles are displayed in Figure 4b with the equivalent circuit model in the inset.It can be seen that the charge-transfer resistance (Rct) of OA-V2C (94.6 Ω) is the smallest among the four samples.The Rct values are estimated to be 269.2,257.4 and 463.2

Figure 1 .
Figure 1.(a) Schematic illustration of the preparation process of OA-V 2 C. SEM images of (b) M-V 2 C and (c,d) OA-V 2 C. (e) Elemental mappings of OA-V 2 C.

Figure 2 .
Figure 2. (a) XRD pattern of the four samples.(b) The survey XPS spectrum of OA-V2C.The corresponding high-resolution spectra of (c) V 2p, (d) O 1s and (e) C 1s.

Figure 2 .
Figure 2. (a) XRD pattern of the four samples.(b) The survey XPS spectrum of OA-V 2 C. The corresponding high-resolution spectra of (c) V 2p, (d) O 1s and (e) C 1s.

Figure 3 .
Figure 3. Galvanostatic charge-discharge profiles at 0.2 A g −1 for (a) M-V 2 C, (b) OA-V 2 C, (c) A-V 2 C and (d) O-V 2 C. The initial three CV curves of (e) M-V 2 C, (f) OA-V 2 C, (g) A-V 2 C and (h) O-V 2 C. (i) Cycling performance of M-V 2 C, OA-V 2 C, A-V 2 C and O-V 2 C at 0.2 A g −1 .

Figure 4 . 12 Figure 4 .
Figure 4. (a) Rate performances of M-V 2 C, OA-V 2 C, A-V 2 C and O-V 2 C. (b) EIS spectra of M-V 2 C, OA-V 2 C, A-V 2 C and O-V 2 C; insert image shows corresponding equivalent circuit model.(c) Longterm cycling performance of M-V 2 C, OA-V 2 C, A-V 2 C and O-V 2 C at 5 A g −1 after 5000 cycles.(d) CV curves of OA-V 2 C electrode at different scan rates from 0.1 to 2 mV s −1 and (e) Log(i) versus log(v) plot.(f) The proportion of capacitive contribution at a scan rate of 0.5 mV s −1 .(g) Capacitive contribution ratios of OA-V 2 C electrode at various scan rates.

Figure 5 .
Figure 5. (a) Ex situ XRD patterns of OA-V 2 C during charge-discharge process.(b) SEM image of the OA-V 2 C after 5000 cycles.
: SEM images of (a) V 2 AlC MAX phase, (b) O-V 2 C and (c) A-V 2 C; Figure S2: High-resolution XPS spectra of (a) V 2p, (b) O 1s and (c) C 1s for M-V 2 C. High-resolution XPS spectra of (d) V 2p, (e) O 1s and (f) C 1s for A-V 2 C. High-resolution XPS spectra of (g) V 2p, (h) O 1s and (i) C 1s for O-V 2 C; Figure S3: Charge-discharge curves of OA-V 2 C anode at different current densities; Table S1: Atomic percentages of V, O, C, F, Na and Al elements in M-V 2 C, O-V 2 C, A-V 2 C and OA-V 2 C based on XPS results; Table S2: Comparison of electrochemical performances between OA-V 2 C and various MXene-based anode materials in LIBs.Author Contributions: Conceptualization, L.G., S.L. and M.C.; Methodology, Y.Z., L.G. and S.L.; Software, Y.Z. and L.G.; Formal analysis, L.G., S.L. and M.C.; Investigation, L.G., S.L. and M.C.; Writing-original draft, Y.Z., Writing-review and editing, Y.Z., L.G., S.L. and M.C.; Supervision, L.G., S.L. and M.C.All authors have read and agreed to the published version of the manuscript.Funding: Key research and development project of Hubei Province (2023BAB036), Key Laboratory of Automotive Power Train and Electronics (Hubei University of Automotive Technology, ZDK1202104), Hubei Provincial Natural Science Foundation of China (2024AFD115), Hubei Key Laboratory of Energy Storage and Power Battery (Hubei University of Automotive Technology, ZDK22023A03), the Doctoral Research Fund of HUAT (BK202324) and National Natural Science Foundation of China (11904091).Institutional Review Board Statement: Not applicable.Informed Consent Statement: Not applicable.